1. Field of the Invention
The present invention relates to a velocimeter, and in particular, to a laser Doppler velocimeter using the Doppler effect.
2. Description of the Related Art
Laser Doppler velocimeters have been used as devices measuring the velocity of a moving object with high accuracy in a contactless manner. The laser Doppler velocimeter irradiates the moving object with laser light to measure the velocity of the moving object using the effect (Doppler effect) in which the frequency of light scattered from the moving object changes (shifts) in proportion to the velocity of the moving object.
In the laser Doppler velocimeter, a detection signal proportional to a velocity obtained through a photodetector is generally based on scattered light. Accordingly, the detection signal has a low level. In addition, the signal-to-noise (S/N) ratio tends to be degraded due to noise superposition.
Additionally, the reflectivity or scattering state of the light depends on the state of the surface of the moving object, so that the signal level fluctuates. Disadvantageously, a Doppler signal may be interrupted during measurement, and thus dropout may occur.
To prevent such a dropout, for example, a method of improving the S/N ratio using a band-pass filter (BPF) and processing a Doppler signal as a continuous signal through a phase-locked loop (PLL) has been used. However, when dropouts frequently occur, the accuracy of measurement decreases due to the frequent occurrence of dropouts.
To overcome the disadvantages, for example, Japanese Patent No. 3423396 discloses a method of detecting light beams scattered from a specific area of a moving object in different directions through a plurality of photodetecting units.
According to Japanese Patent No. 3423396, a selecting unit is used to reduce the dropouts of detection signals of these photodetecting units. As disclosed in Japanese Patent No. 3423396, the selecting unit processes the detection signal of each photodetecting unit as it is in analog form.
Processes necessary for higher accuracy, (e.g., inter-signal phase shift processing, noise processing, and high time resolution processing on the detection signals of the photodetecting units), are difficult to perform using an analog circuit.